The reaction of a phosphonitrilic halide and an alcohol in the presence of pyridine is known. According to Dutch Patent Publication 71/06772, hexa-alkoxy phosphazene is prepared in a known manner by reacting phosphorus pentachloride and ammonium chloride in the presence of a solvent and after separation of the solid components the reaction mixture is reacted with a univalent alcohol and pyridine. The resultant pyridine hydrochloride is treated with lye to produce a pyridine water fraction from which the pyridine is regenerated. The organic fraction remaining after separation of the aqueous pyridine fraction is separated into the alkoxy phosphazene and a solvent-rich liquid phase. The difficulty with this process is that in the preparation of the alkoxy phosphazene and the purification of pyridine different solvents are used making necessary separate regenerations. According to the invention in the Dutch Patent Publication, chlorobenzene is used as the reaction medium for producing hexachlorophosphazene and the alkoxyphosphazene. The pyridine is recovered from the aqueous fraction by extraction with the solvent-rich liquids after separation of the alkoxyphosphazene, followed by distillation of the resultant extract. Thus, auxiliary agents such as the pyridine acid fixing agent and solvents are regenerated and used for recycle.
In a paper by Dishon, Journal of the American Chemical Society, Volume 71, p. 2251 (1949), cyclic trimeric phosphonitrilic chloride in pyridine was reacted with butyl alcohol at 0.degree. C with vigorous stirring to produce the dibutylester of phosphonitrilic chloride. Further, Audrieth et al in a paper in Chemical Review, Volume 32, pp. 129-130 (1943) discloses the reaction of phosphonitrilic chlorides with alcohols in the presence of pyridine. However, no indication of the method of reaction or the conditions of reaction were indicated in this paper. Audrieth et al refers to Wissemann who used alcohols with or without pyridine as condensing agent at elevated temperatures. However, a number of possible reactions were believed to take place in this type of reaction which would lead to undesirable side reactions, particularly the reaction of the phosphonitrilate ester with HCl to produce P-O-H bonds. These species increase water solubility and hence are undesirable in a flame retardant incorporated in a material which is subject to numerous launderings.
From the previously described prior art, the processes disclosed appear to produce only the ordinarily expected fully substituted or fully esterified phosphonitrilate polymers, e.g., hexapropoxyphosphazene or hexapropoxy phosphonitrilate polymers. In contrast, according to the present invention there is believed to occur a simultaneous substitution or esterification and condensation whereby products are produced having entirely different properties, insofar as chemical and physical properties, as well as, advantageous, and superior qualities for flame retardant applications, from prior art process materials.
As far as can be determined, the process of the present invention has not heretofore been disclosed or recognized for producing products of superior efficacy for fire retarding cellulosic materials. According to this invention, a phosphonitrilate polymer is produced which exhibits a relationship of viscosity to molecular weight which is surprising from a knowledge of prior art phosphonitrilate polymer and which is believed to provide superior retention and flame retardance in the regenerated cellulose or rayon fiber at concentrations lower than previously considered practical.